Interactions between surface conditions, the Mediter-ranean Sea, and cave climate within two littoral caves in Mallorca: implications for the formation of phreatic overgrowths on speleothems

Phreatic overgrowths on speleothems from Mallorca’s littoral caves are valuable markers of former sea-level stands. These carbonate encrustations form as CO2 degasses from brackish cave water that is hydrologically connected to the Mediterranean Sea. This study uses time series analysis to document relationships between surface conditions of temperature, barometric pressure, precipitation, tidal level of the Mediterranean Sea, and the coastal caves’ microenvironment of temperature, partial pressure of CO2 (pCO2), and water level to contextualize overgrowth formation in Cova des Pas de Vallgornera (Vallgornera) and Coves del Drac (Drac). Water level in both caves was an attenuated semidiurnal function of Mediterranean Sea level with a lag of about four hours. The impact of individual rainfall events on cave water level was negligible during the study period. pCO2 of cave air at both sites reached an annual maximum in September and decreased rapidly when surface air temperature fell below the cave air temperature (mean ~19 8C). As this threshold was reached, cooler and denser tropospheric air descended into the caves, initiating cave ventilation and displacing high-pCO2 cave air that had accumulated. Observed pCO2 was lower in Drac than in Vallgornera, and had small daily fluctuations because of bigger passages, fewer constrictions, and a large collapse entrance. The frequency and magnitude of pCO2 fluctuations were higher in Vallgornera than in Drac, with both caves showing twicedaily water level maxima causing displacement of high-pCO2 air from the cave alternating with water level minima causing tropospheric air to enter the cave via a ‘‘piston effect.’’ A secondary control on pCO2 variation can be attributed to variation in tropospheric barometric pressure. Thus, the geochemical conditions favorable for overgrowth formation are, in part, the result of this tidally-controlled cycle of cave-water level. The cycle causes cavetroposphere air exchange that drives CO2 degassing, and therefore, the formation of phreatic overgrowths on speleothems.

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